Reducing reflection of light from surfaces, and articles so produced



Search Room A ril 28, 1942. c. H. CARTWRIGHT EI'AL 2,281,475 REDUCINGREFLECTION OF LIGHT FROM SURFACES, AND ARTICLES 50 PRODUCED Filed Aug.4, 1959' 7 i /0 leef/eciance Air/1} 1 M fima+% Patented Apr. 28, 1942UNITED STATES PATENT OFFICE REDUCING REFLECTION OF LIGHT FROM SURFACES,AND ARTICLES SO PRO- DUCED Application August 4, 1939, Serial No.288,458

6 Claims.

This invention relates to the art of materially diminishing thereflection of light from surfaces, and is concerned more particularlywith the treatment of a surface of a light-transmitting article (e. g.,a plate made of glass, a lens, a prism, or the like) wherebysubstantially to eliminate, or at least very materially reduce, thereflection of light from such surface. The invention is concerned alsowith improved light transmitting articles which have been treated inaccordance with the process hereinafter disclosed and claimed.

In our co-pending application entitled Process of decreasing reflectionof light from surfaces and articles so produced, Serial No. 247.974,filed December 27, 1938, there is described a method of reducing thereflection of light from the surface of an optical element whichcomprises applying to the surface a layer of a suitable substance havingan effective index of refraction intermediate the index of refraction ofthe optical element and the index of refraction of air and having anoptical thickness approximately :c/4 of the wave length of preselectedlight, a: being a positive odd integer not greater than 9 and preferablybeing 1. It was there disclosed that a film of lithium fluoride, sodiumfluoride, sodium aluminum fluoride, calcium fluoride, or the like, ofthe appropriate optical thickness and appropriate index of refraction,on a surface of a glass article very greatly diminishes light reflectionfrom said surface.

It has been observed that with age these films become harder and moreresistant to abrasion without appreciable alteration in theeffectiveness of their diminution of reflection. This hardening effectis exhibited to a marked degree in the case of films of magnesiumfluoride, such films becoming noticeably harder in the course of twoweeks aging at normal temperatures.

It has been found desirable to bake the clean surface to be treated atan elevated temperature of 400 to 450 C. for perhaps an hour to improvethe tenacity of the film to the glass and also the ruggedness of thefilm itself.

We have now found that the ruggedness of such films may be improved verymaterially by subjecting the filmed article to an appropriate bakingtreatment. Thus, we have found that if we subject a glass plate,carrying on a surface thereof a metal fluoride film or an alkali metalfluosilicate film (of the above defined characteristics), e. g., amagnesium fluoride film, a calcium fluoride film, a potassiumfluosilicate film, a sodium fluosilicate film, or the like, to heatingat elevated temperature,-e. g., to baking at a temperature of from about350 to about 500 C.,- for some time, e. g., for a period of time of theorder of from several minutes to about an hour and a half or more, thefihn is rendered desirably tenacious and rugged.

That is to say, the so-baked filmed surface is not disturbed bycleansing with water and soap andmay be so rugged that it cannot bescratched with the finger-nail. The baking treatment does notappreciably alter the reflection-diminishing property of the film. MgFzbaked for seven hours at 365 C. was equally improved.

Illustrative examples of the principles of th present invention are thefollowing:

Example I The starting material was a glass plate carrying on itssurfaces films of magnesium fluoride applied as described in ourapplication Serial No. 247,974. The index of refraction of the glass was1.52; the effective index of refraction of the magnesium fluoride filmwas 1.37. Each film had an optical thickness of approximately 1400 A.The filmed glass plate exhibited for the two surfaces 2.2% reflection ofmonochromatic light having a wave length of 5500 A (as compared with 8%for untreated glass) and substantially reduced reflection throughout thevisible spectrum.

The filmed plate was placed in an electric oven, and the oven wasbrought up to 420 C, within a period of 1.5 hours. The oven was thenmaintained at 420-440 C. for from 1.5 to 2.0 hours, after which the ovenwas allowed to cool (cooling roughly 3.0 hours). As a result of thebaking treatment the magnesium fluoride film was so indurated and weldedto the glass that it could not be scratched with the finger-nail andcould be scrubbed with soap and water without the surface being injured.After baking, the fihns each had an effective optical thickness of 1150A. The baked product showed a reflectivity of 1.8% at 4600 A, and areflectivity of 2.2% at 5500 A; throughout substantially the wholevisible spectrum the reflectivity was reduced. The net effect of thebaking treatment in this specific example, from the reflectancestandpoint, was an appreciable improvement in diminution of reflection.Percent reflectance values ranged as follows: at 4000 A, 2%; at 4500 A,1.8%; at 5000 A, 1.9%; at 5500 A, 2.2%; at 6000 A, 2.7%; at 6500 A,3.1%; and at 7000 A, 3.5%.

Example If In this example we in general duplicated the conditionsdescribed in Example I with the exception that the article consisting ofa flint glass plate having a refractive index of 1.69 and carryingmagnesium fluoride films on its surfaces was baked at 400 C. for 35minutes. The films, before the baking, had optical thicknesses ofapproximately 1400 A each.

The so-baked magnesium fluoride films were very materially more ruggedthan they were prior to the baking; they could be washed with soap andwater without injury to their surfaces, and resisted scratching with thefinger-nail.

Thereafter the baked article was washed with a solution of stearic acidin benzene (1 part by weight of stearic acid in 1000 parts by weight ofbenzene), washed with pure benzene and dried. This subsequent treatmentof the baked films was found to enhance the water-resistance of thefilms, making the latter unaffected by water; the subsequent treatmentdid not adversely afiect the ruggedness of the baked magnesium fluoridefilms.

The effects of the above specifically described baking and subsequenttreatments (Example II) on the reflectance of the magnesium fluoridefilmed glass plate are illustrated in the accompanying drawing, thesingle figure of which is a curve diagram, in which curve A representsthe percent reflectance of the filmed plate before the baking treatment,curve B represents the percent reflectance of the filmed plate after thebaking treatment, curve C represents the percent reflectance of thefilmed plate after the baking and subsequent washing with stearic acidsolution, and curve D represents the percent reflectance of theuntreated glass plate (i. e., of the glass plate before being providedwith the magnesium fluoride films over its surface). It will be seenthat the net effect of the baking treatment, from the reflectancestandpoint, was an appreciable improvement in diminution of reflection.

We have found that substantially the same improvements in ruggedness andresistivity to abrasion may be gained by baking the films-bearingarticle at a somewhat lower temperature but for a correspondinglyincreased period. Thus, a seven hour baking of the magnesium fluoridefilms at 365 C. appeared to result in a hardening effect equal to thatproduced by baking at 420-440 C. for 1.5 hours.

This baking treatment was found to have simi lar ruggedness-impartingeffects on cryolitefllmed glass. In the case of the cryolite film, thecoating of the baked product, after repeated washings with water andsoap, showed some slight blemish but was, nevertheless, materially morerugged and resistant to abrasion than a similar but unbaked cryolitefilm. The same baking treatment, applied to a glass plate carrying afilm of calcium fluoride, was found to yield a product about as ruggedas the above-described baked magnesium fluoride film.

It was observed that by baking a LiF film at about 550 C. the film,while fully adherent, was milky in appearance and gave a difiusedeffect.

As was suggested above, films, of appropriate optical thickness, ofsodium fluosilicate or potassium fluosilicate, are particularlyadvantageous for the provision of optical elements of reduced lightreflection. These salts have very low indices of refractionin theneighborhood of 1.29 to 1.30. Moreover, the potassium fluosilicate,because of its low solubility in water, is a very desirable coatingmaterial. Films of either of these substances may be improved inruggedness by the baking treatment of the present inventio'n.

Minimum and maximum baking temperatures and times for all operablecoating materials may be determined in accordance with the foregoingdescription.

This application contains subject-matter in common with our applicationSerial No. 247,974, filed December 27, 1938.

We claim:

1. Process of improving the ruggedness and tenacity of areflectance-diminishing light-transmitting layer of solid and stablemetallic fluoride on a surface of a solid light-transmitting article,said layer having an optical thickness approximately one-fourth thewavelength of pre-selected light and an effective index of refractionapproaching the square root of that of the article, which comprisesbaking the layer on the article at a temperature between about 300 andabout 500 C. at which the optical properties of the layer and of thearticle are substantially maintained and for a period of time suflicientto effect a substantial improvement in the ruggedness and tenacity ofsaid layer.

2. Process of improving the ruggedness and tenacity of areflectance-diminishing lighttransmitting layer of magnesium fluoride ona surface of a solid light-transmitting article, said layer having anoptical thickness approximately one-fourth the wavelength of preselectedlight and an effective index of refraction approaching the square rootof that of the article, which comprises baking the layer on the articleat a temperature between about 300 and about 500 C. at which the opticalproperties of the layer and of the article are substantially maintainedand for a period of time sufficient to effect a substantial improvementin the ruggedness and tenacity of said layer.

3. Process of improving the ruggedness and tenacity of areflectance-diminishing lighttransmitting layer of cryolite on a surfaceof a solid light-transmitting article, said layer having an opticalthickness approximately one-fourth the wavelength of preselected lightand an effective index of refraction approaching the square root of thatof the article, which comprises baking the layer on the article at atemperature between about 300 and about 500 C. at which the opticalproperties of the layer and of the article are substantially maintainedand for a period of time sufficient to effect a substantial improvementin the ruggedness and tenacity of saidlayer.

4. Process of improving the ruggedness and tenacity of areflectance-diminishing lighttransmitting layer of alkali metalfluosilicate on a surface of a solid light-transmitting article, saidlayer having an optical thickness approximately one-fourth thewavelength of preselected light and an effective index of refractionapproaching the square root of that of the article, which comprisesbaking the layer on the article at a temperature between about 300 andabout 500 C. at which the optical properties of the layer and of thearticle are substantially maintained and for a period of time sufficientto effect a substantial improvement in the ruggedness and tenacity ofsaid layer.

5. Process of improving the ruggedness and tenacity of a.reflectance-diminishing lighttransmitting layer of potassiumfluosilicate on a surface of a solid light-transmitting article, saidlayer having an optical thickness approximately one-fourth thewavelength of preselected light and an effective index of refractionapproaching the square root of that of the article, which comprisesbaking the layer on the article at a temperature between about 300 andabout 500 C. at which the optical properties of the layer and of thearticle are substantially maintained and for a period of time sufiicientto effect a substantial improvement in the ruggedness and tenacity ofsaid layer.

6. An article comprising a solid light-transmitting body portion and ona normally partially light-reflective surface of said body portion a.baked-on coating consisting of a light-transmitting layer of a solid andstable metallic fluoride having an index of refraction approaching thesquare root of that of the material constituting said body portion andan optical thickness approximately one-fourth the wavelength of saidlight, said layer functioning materially to reduce reflection of lightfrom said surface, the product being characterized by such ruggedness ofthe metallic fluoride layer and such adherence of said layer to thearticles surface, produced by subjecting the coated article to a bakingtreatment at a temperature between about 300 and about 500 0., that thelayer-coated surface withstands cleansing with water and soap withoutmaterial blemish to said layer.

CHARLES HAWLEY CAR'I'WRIGHT. ARTHUR FRANCIS TURNER.

